It's my favourite of theirs but they are legitimately a band that have sounded different with each album and I think the reason why a lot of people love them so much is that their progression as a band matches their progression as people.
First album: Pretty much a straight-up teen emo album (with flashes of what was to come) that people got into because they were emo teens
Second album: Much more grown-up, cohesive version of the first album's sound with better, wittier lyrics and much more mature song structures etc, matches fans of the first album having grown up a bit.
Third album (TDAG): Fucking outstanding straight up rock album which would be judged a lot differently by non-emo audiences if it was a debut.
Fourth album: Doesn't really sound like any of the ones that came before it. Sounds almost like a hardcore band in places but also experiments with some really funky basslines and distorted vocals and stuff.
Each of those albums came out at a time where they hit the exact sweet-spot for me (and loads of other people my age I'd imagine) for what I wanted from a guitar band. That in turn means I've never fallen out of love with any of the albums that came before them and they simultaneously hold nostalgic value as well as loving them in their own right.